atmospheric stability

  Adiabatic means that there is no heat exchange or flux between an air bubble and its surroundings. Adiabatic temperature change is the change in temperature due to expansion/compression as an air parcel rises/descends. The rate at which temperature decreases in a rising, expanding air parcel is called the adiabatic lapse rate. If a rising air parcel remains dry with a relative humidity below 100%,it will decrease in temperature at a rate of about 10C/km. This is known as the dry adiabatic lapse rate. One difference between the dry and saturated adiabatic lapse rates is that whereas the former remains constant the later varies with temperature. This is because saturated adiabatic lapse rate depends upon the amount of heat released by condensation which in turn depends upon the moisture content and therefore the air temperature. Contrast between the environmental lapse rate and the adiabatic lapse rates controls the stability of the atmosphere.

  There are three types of stability. The first one is absolute stability. Stable atmospheric conditions prevail when the environmental lapse rate is less than the saturated adiabatic rate. An example of this condition is shown in figure 1. at the surface both the parcel of air and the air of the surrounding environment have the same temperature. The surrounding air is changing its temperature at a rate of 6.5C/100meters. The parcel on this day is dry and will rise and cool at a temperature of 1C/100 meters. After giving the parcel a slight upward push, it rises to a level of 1000meters where it cools to a temperature of 20C. A measurement of the air surrounding the parcel shows a temperature of 23.5C. In other words, the parcel is colder and denser than the surrounding air at 1000 meters. If the uplift mechanism ceased, the parcel of air would return to the surface.

  Air is unstable when the environmental lapse rate is greater than the dry adiabatic rate. Under these conditions, a rising parcel of air is warmer and less dense than the air surrounding it at any given elevation. Figure 2 depicts unstable conditions. Follow up the graph for the rising parcel of air. Note that at any elevation above the surface the parcel temperature is higher than the air that surrounds it. Even as it reaches the dew point temperature at 2000 meters, the air remains warmer than the surrounding air. As a result, it continues to rise and cool at the saturated adiabatic rate. Vertically developed cloudes are likely to develop under unstable conditions such as this.